There are many well-known uses for panel displays, among them television screens, including very small screens, such as for "wristwatch" TVs, and familiar computer screen applications. Computer systems require user input to initiate functions and to provide values for variables, among other reasons, and typically have displays, also called video display terminals (VDTs), for providing data and information to the user.
There are several different technologies used for displays, among them cathode ray tubes (CRTs), liquid crystal displays (LCDs), vacuum florescent displays (VFDs), gas discharge displays, electroluminescent displays (ELDs), light-emitting diode (LED), incandescent displays, and electromechanical displays. The most used display technology for computers is the well known CRT, which is used with almost all desktop VDTs. Other display types are used for various purposes. For example, LCDs are common in many digital wristwatches.
While CRTs are the most commonly used displays for VDTs, they are not well suited for portable computer displays such as laptop and notebook types. CRTs are too bulky and generally too fragile for use in small portable units that must withstand transport and occasional shock. CRTs are completely out of the question for small displays, such as "wristwatch" TVs, because of their size and complexity.
For flat panel displays for portable computer systems and other uses, liquid crystal technology is widely used, and some commercially available products use gas plasma displays, which are more expensive and require high voltage drives. Another type coming into wider use is electroluminescent displays (ELDs), which use areas or layers of material that emit light under the influence of an electrical field. The ELDs typically require high voltage as well, such as 150 to 200 volts.
There are problems and concerns common to all types of available flat panel displays, among them intensity of light output, power consumption, voltage required for operation, and resolution. Portable computers and portable TVs are intended for use outside the usual office or home environment, where there is little control of ambient light. It is desired that these be useful even in bright sunlight. Light output, (intensity), therefore, is a very big concern. A display that has poor light output cannot provide good visibility and contrast for images, especially under conditions where the ambient lighting is relatively strong.
Some displays, such as LCDs, are passive and have no inherent light generation ability at all. These rely on auxiliary light supplied, such as backlighting and by reflection.
It is generally true for light-emitting displays that more light can be delivered at the expense of power consumption, and power consumption for portable displays, such as for portable computers is a very serious concern. Every effort is normally made to minimize power consumption, to provide the maximum possible usable time between necessary battery charge or replacement. High power consumption also develops more heat, and dissipation of heat can be an additional problem.
Resolution becomes more and more of a concern as the overall size of a display becomes smaller. For example, one of the operating modes of the popular VGA video adapter for computer screens provides 640 pixels per line and 480 lines. A pixel, for this purpose, may be thought of as a "light dot". This is a total for the screen of 307,200 pixels. This is about 6 pixels per square mm for a screen of about 200 mm by 250 mm. The distance between pixels is about 300 microns in this arrangement. A micron is 10.sup.-6 meters.
A "wristwatch" TV may have a display as small as about 3/4 inch (about 20 mm) square. This is about 400 square millimeters, and at 6 dots per square mm, a total of 2400 pixels to form the same images displayed on a VGA computer screen 100 times larger in area. The resulting images must be very rough, and alphanumerics would not be displayable.
What is needed is a display that significantly increases light output for power consumed, and does so with a lower voltage drive than the 150 to 200 volts required of some displays today. The need is to enhance visibility and contrast even with lower power use, and at the same time to provide a dot density sufficient for very small displays.